A computer system typically requires large amounts of secondary memory, such as that provided by a disk drive, to store information (e.g. data and/or application programs). As the performance of computers and associated processors has increased, disk drive performance has increased. For example, data and track density increases, magnetic media improvements, and greater numbers of heads and platters in a single disk drive result in higher data transfer rates.
Large disk drives can be very expensive, and thus lead to additional high costs when replacing a drive to obtain greater capacity or performance. Single drive systems can also have limited redundancy of stored data. Damage or replacement of a system can cause a temporary loss of access to storage services or a permanent loss of data.
Multiple drives can be coupled together in parallel to mitigate the disadvantages of single disk drive systems. Data files are broken into chunks that may be accessed simultaneously from multiple drives in parallel. One such system of combining disk drives in parallel is known as “redundant array of inexpensive/independent disks” (RAID). A RAID system can provide greater storage capacity than a large single disk drive system, but at lower cost, greater reliability and with high data transfer rates due to the parallelism of the array.
Blocks of data are written to a RAID system in the form of stripes, where a stripe includes one storage block on each disk drive in an array of disk drives in the system. Data blocks can be read simultaneously from each drive, to provide fast data accesses for large files. RAID systems also allow incremental increases in storage capacity through the addition of disk drives to the array. When a disk fails in the RAID system, it may be replaced without shutting down the entire system. Data on a failed disk may be recovered using error correction techniques.
A RAID system typically views its associated collection of disk drives as a collection of stripes in one or more groups of disk drives. A group of drives is commonly called a “RAID group”. A stripe is typically implemented with a pre-defined geometry within a group. That is, each block in a stripe is located at the same offset (disk block number) relative to its disk.
RAID systems are typically defined in terms of six basic configurations, referred to as RAID level 0 through RAID level 5. Each RAID level has advantages and disadvantages. For example, in a RAID level 4 system, data comprising a plurality of blocks are stored on N data disks in a “stripe” of N storage blocks. RAID level 4 adds an extra parity disk drive containing error-correcting information for each stripe in the system.
Stripe writing presents conflicts for both file systems and disk drivers. File systems typically prefer to allocate space on a single drive for storage of related data, like that derived from a single file. This permits “read-ahead” behavior to reduce file access time by anticipating future requests. Disk drivers tend to be optimized to take advantage of the physics of disks, such as rotation rates and read/write head access times. Again, storage of related data blocks on the same disk drive is advantageous.
RAID systems tend to view disk storage blocks in terms of stripes, and handle write requests in terms of a stripe. Typically, data blocks are accumulated to create a single stripe write to reduce parity calculation overhead. When a RAID disk array manager processes stripe writes, related blocks of data can be scattered over multiple drives.